Phantom fat is hiding in your cereal bowl. It's the bad boy in your bag of microwave popcorn. It lurks in those low-fat cookies and even in that energy bar.

The stuff is called trans fat, created when ordinary vegetable oil is processed into partially hydrogenated oil. It's why Crisco stays solid at room temperature and what makes cakes moist, cookies fresh and crackers crisp. Partially hydrogenated oil is in about 40 percent of the food at the grocery store, including some products most consumers regard as healthy.

A generation ago, when cardiologists waved Americans off saturated fats like butter and beef tallow, partially hydrogenated oils became a preferred alternative. Now, in an about-face, researchers have determined that trans fat can grease the way to a heart attack faster than a cup of lard.

Some of the nation's leading medical researchers, including many in the Bay Area, also believe that the trans fat that marbles the modern American diet may be why kids are so fat, diabetes is at record levels and why some people develop cancer. They say trans fat is a big player in Syndrome X, a cluster of health problems characterized by a beer belly, high blood pressure and out-of-whack blood fats and sugars.

"There should be a warning on food made with this stuff like there is on nicotine products. It's that bad for you," says Dr. Jeffrey Aron, a University of California at San Francisco professor of medicine and one of the nation's leading experts on fatty acids and their effects on the body.

But there is no warning label. Trans fat amounts aren't regulated at all, so manufacturers and fast food operators don't have to list it on nutrition labels. That means there's no easy way to know how much you're eating.

And chances are, it's a lot more than you think.

Virtually every fast-food or family restaurant french fry is cooked in trans fat-filled grease. Almost half of all cereals, both cold and hot, contain it, according to the Food and Drug Administration. So do 70 percent of cake mixes, 75 percent of chips and other salty snacks, 80 percent of frozen breakfast foods like waffles, and 95 percent of cookies.

Even products people buy when they want to eat healthier -- granola, power bars and low-fat cookies and crackers -- are made with partially hydrogenated vegetable oil. During a recent informal survey of 140 varieties of crackers on a typical supermarket shelf, only three brands had no partially hydrogenated oil.

Because trans fat flies under the radar, food labeled "low in saturated fat," "cholesterol-free" or "made with 100 percent vegetable oil" can have so much trans fat that consumers focused on heart-healthy food wouldn't touch many of these products -- if they knew.

The FDA, which could decide by September whether to require trans fat on food labels, estimates that listing it could prevent as many as 5,600 heart disease deaths a year -- not only because people would be able to choose healthier foods but also because manufacturers would choose to reduce the amount of trans fat rather than put it on the label.

"The labels on food say how much fat but not what kinds of fat," Aron explains. "It's insidious and we're nowhere near the level of awareness we need to be."

WHAT IT IS

Trans fat is the byproduct of the hydrogenation process patented in 1903 by chemist William Normann. He discovered a way to turn relatively healthy liquid vegetable oil into something that stays solid at room temperature and improves shelf life. However, it also blocks arteries just as readily as saturated fat. Crisco was soon introduced with an advertising campaign that called it "a scientific discovery which will affect every kitchen."

But partially hydrogenated oil didn't really catch on until World War II, when people turned to margarine and shortening as alternatives to rationed butter. As convenience foods started to hit the market in the 1960s and '70s, more hydrogenated oil was used.

Consumption rose significantly in the 1980s and 1990s when the public embraced the connection between heart disease and saturated fats. Fast food restaurants replaced the beef fat in the fryers with partially hydrogenated oils. Food manufacturers began to tout products that were free of saturated fats but that had plenty of trans fat instead.

Now, in light of new research, those changes don't seem like such great ideas. An American Heart Association study released last July showed that foods cooked with trans fat might clog arteries quicker than food cooked in animal-based saturated fat.

Like beef fat, trans fat also raises the level of bad cholesterol (LDL), which can lead to strokes and heart attacks. But trans fat goes one step further, scrubbing away good cholesterol (HDL) that keeps arteries clean.

SYNDROME X

The ongoing Nurses' Health Study of 80,000 women, conducted by Harvard Medical School and the Brigham and Women's Hospital in Massachusetts, showed that for each 2 percent increase in the amount of calories from trans fat, a woman's coronary risk jumps by 93 percent. And the New England Journal of Medicine reported in 1998 that women who want to reduce their risk of heart disease would be better off if they replaced saturated and trans fats with unhydrogenated mono- or polyunsaturated fats than if they cut down on the total amount of fat they eat.

But heart disease may be only one of trans fat's threats to health. A growing number of doctors say it plays a starring role in something more prevalent and ultimately worse -- Syndrome X.

Former Stanford University director of endocrinology Dr. Gerald Reaven named Syndrome X in 1988 after he observed a collection of health problems ultimately linked to cells' inability to process insulin. Also called metabolic syndrome or, more commonly, beer belly syndrome, the prevalence of the condition has increased along with the amount of refined foods and partially hydrogenated oil Americans eat.

"Forty years after it's been in the food system on such a large scale, what is becoming clear is that this is dangerous stuff," says Jack Challem, a Tucson-based nutrition expert and author of "Syndrome X: The Complete Nutritional Program to Prevent and Reverse Insulin Resistance."

Even though Americans' saturated fat intake has dropped by 10 percent across the board, diabetes rates and obesity have gone up, researchers say. The cases of diabetes alone climbed 33 percent in the 1990s, and doctors point to lifestyle changes as part of the problem. It's not so much Crisco in pie crusts that's raising trans fat levels, but rather prepared foods and popular fried restaurant food. Average Americans now get a third of their calories from food they haven't prepared themselves. The nation's children eat 40 percent of their meals at fast food restaurants.

CLOGGING THE WORKS

Trans fatty acids make up a small part of the average daily diet -- somewhere between 3 to 8 percent of the total daily caloric intake. But even a handful of grams a day is enough to gum up the workings of a cell, says Aron, author of "Gut-Check: Your Prime Source for Bowel Health and Colon Cancer Prevention."

Picture the cell as a Swiss watch, he says. Sprinkle a few very fine grains of sand in that watch and it will continue to tick, but after awhile it won't keep time. Eventually it won't work. That's how trans fat works in the body, he says. It changes how the cell membranes work -- how they talk to each other and function. Trans fat can help make cells resistant to insulin, and when you have resistance to insulin you have obesity.

Researchers at the Centers for Disease Control earlier this month released estimates that showed at least 47 million Americans, more than 20 percent, have Syndrome X. Other Syndrome X experts say that estimate is conservative and put the figure as high as 70 million.

Trans fat is such a hidden part of the American diet, people have no idea that it contributes to their illness.

"One of the defining moments for me came when I looked at a box of breakfast bars," says nutrition expert Challem, who urges people to read labels for clues to hidden trans fat (see sidebar). "This is in everything. Trans fatty acids are like a wild card. It's as if you're screwing up how the body processes food."

HOW DO YOU KNOW?

The fight to put trans fat on food labels has been a difficult one, in part because adding trans fat would be the first change to the national nutrition facts panel since labels became mandatory in 1993.

A NEW BREED OF OIL

Meanwhile, food manufacturers are rushing to find new ways to deliver the taste people want in baked goods and snacks without using as much trans fat.

"As the science has evolved related to trans fat, of course the industry is looking to different sources and re-engineering and reformulating products," says Bob Earl, senior director for nutrition policy and regulatory affairs for the National Food Processors Association.

New, lower-fat margarines are being marketed as trans fat-free. Oil processors are mixing super-hard, trans-free hydrogenated oils with liquid oil to make a suitable replacement. Nutritionists are revisiting tropical palm and coconut oils, which may not be as bad as once thought and could actually have cancer-fighting properties. Healthier canola and sunflower oils that remain stable at high temperatures are coming onto the market. New seed oil crops are being bred to produce oils that don't need hydrogenation.

"Biotechnology holds great promise for us," Earl says.

Some natural food companies, like Barbara's Bakery and Newman's Own Organics, have begun to use alternatives to hydrogenated oil. Peter Meehan, CEO of Newman's Own Organics, says palm oil has almost half the saturated fat of regular palm kernel oil and is a breakthrough for baked products that have required partially hydrogenated oils. Finding that substitute was key to Newman-O's, the company's chocolate sandwich cookie, introduced last year.

Still, eliminating so-called "bad fat" is likely impossible for a nation with a taste for shelf-stable cakes, crunchy snacks and fast food.

"If you were to get rid of saturated and hydrogenated fats, bread would not have the same flavor, crackers wouldn't have the crumble, pie crusts wouldn't be tender. We need these fats to maintain taste and performance," Earl notes.

He and others in the food business warn against trans-fat hysteria in a culture where exercise has declined and fast food consumption has increased. In other words, don't blame the food for a fatter, lazier public.

It's an intersection where food manufacturers and some health advocates actually agree.

"At this point I'm not convinced trans fat is poison people shouldn't eat. They should just eat less of it," says Wootan. "I would hate to see them get trans fat out of their diets at the risk of raising saturated fat levels again."

On the standard American food label, trans fat content is invisible. Only three types of fat -- unhealthy saturated fat and, in cases when certain health claims are made, poly- and monounsaturated fats -- must be listed under the total fat content.

But there are some tricks to figuring out if food has trans fat.

-- -- Figure out how much fat you need every day. For an average healthy person who eats 2,500 calories a day, about 30 percent or less should come from fat. That translates to about 80 grams a day. And of that, only about 25 grams should be saturated or trans fat.

-- Look for the words hydrogenated, partially hydrogenated or fractionated in the list of ingredients. Trans fat comes from hydrogenation. The higher up partially hydrogenated oil is on the list of ingredients, the more trans fat the product has.

-- Note the amount of total fat listed and compare it to the breakdown of specific fats on the label. The results may surprise you. A box of reduced-fat Triscuits, for example, has 3 grams of fat per 7-cracker serving. Saturated fats make up 1/2 gram and monounsaturated fats 1 gram. The crackers have no polyunsaturated fats, so the remaining 1 1/2 grams must be the only other kind of dietary fat -- trans fat.

One study, by the Center for Science in the Public Interest, showed that foods with partially hydrogenated oils in the ingredient list contained 1 gram of hidden trans fat for each gram of saturated fat. That means that Chips Ahoy cookies, for example, with 2 grams of saturated fat per serving also contain 2 additional grams of trans fat.

Know your fats

There are only four kinds of fats in our diet -- monounsaturated, polyunsaturated, saturated and trans.

-- -- Mono- and polyunsaturated fats. These are considered "good" fats. They do not clog arteries and, in moderation, can contribute to a healthy diet. They include olive, canola, peanut and walnut oils.

-- Saturated fat. This is mainly animal fat, the kind found in beef, butter, lard, the skin of chicken, whole milk, whipped cream, egg yolks and other products that come from animals. Coconut and palm oils are also saturated. Too much raises the level of artery-clogging cholesterol.

-- Trans fat (trans-fatty acids). These are formed when oil is hydrogenated. Some naturally occurring trans fats can be found in small amounts in animal products. Like saturated fat, trans fat raises the level of harmful blood cholesterol (LDL) as well as the ratio of LDL to the more beneficial HDL cholesterol. Some researchers believe it changes how cells process insulin -- which can lead to diabetes -- and have linked it to cancer. Trans fat is found in many processed, convenience and fast foods -- french fries, fried chicken, doughnuts, pastries, cookies, crackers and some breakfast cereals.

-- Partially hydrogenated oil. This manufacturing process creates trans fat. A hydrogen atom is mixed with non-saturated liquid oil from plants like corn or soy to make fat such as shortening and margarine that stay solid at room temperature.

-- Fractionated oil. This type of oil is created by a manufacturing process that uses high temperatures or solvents to separate hydrogenated oil into liquid and solid parts. When listed on food labels, it indicates the presence of trans fat.

Two brands top trans fat-free margarine rating

To most cooks, and certainly to The Chronicle Food staff, margarine never has been a good substitute for butter. But there comes a time when the saturated fat in butter needs to be avoided. And now, it turns out that trans fat -- the stuff that makes most margarines stay solid at room temperature -- is worse for the body than saturated fat.

As a result, new lower-calorie, trans fat-free margarines are hitting the market. For people who have made a decision to eat less of both kinds of bad dietary fat, the Food staff tasted eight of them. The bad news is, most were so bad that we can't recommend them. The good news is we found two to recommend. Keep in mind that these are margarines we suggest for people who need to restrict their intake of saturated and trans fat.

-- SMART BALANCE (16 OUNCES, $1.99 at many markets). This was the best of the lot. It contains 80 calories per tablespoon, along with 2.5 grams of saturated fat. The selling point is its lack of trans fat and a claim that the balance of good to bad fats makes it better for blood cholesterol. It had the mildest, least oily taste. It is a very pale product and very stiff, which put some tasters off. But the clean, non-oily taste made it a winner.

-- BRUMMEL & BROWN (16 OUNCES, $1.89 at many markets). This spread was the second favorite. Made with yogurt to give it a slight dairy tang, it has 45 calories per tablespoon and one gram of saturated fat. The texture is very creamy and the flavor just slightly salty. It is a lower-fat alternative to Smart Balance.

If you want a trans fat alternative that tastes more like butter, make up a batch of "Better Butter." The recipe below is adapted from a recipe by Laurel Robertson, author of "Laurel's Kitchen" (Bantam Books, 1981). It does contain saturated fat from the butter, but can be a good alternative for people who don't want to use a processed spread.

This keeps well in the refrigerator. Oil and butter alone will work, but the other ingredients help it stay firm longer. Even when cold, it spreads easily. It liquefies if left at room temperature, but will solidify when refrigerated.

Partially hydrogenated oils are more prevalent than you might think. Since only total fat and saturated fat are required to be on food labels, you have to be a fat detective to figure out if trans fat is in the product. A trip through a supermarket and a quick read of ingredient lists found the surprises below.